With development of data storage technology, various data storage devices are capable of providing a user with a considerably increased data storage capability, and a data accessing speed is greatly improved. In a storage system, read and write speeds are key factors for determining the performance thereof. It usually takes different time to perform read and write operations, which varies with the different storage media. In a traditional mechanical disk (e.g., a magnetic disk, HDD), a speed difference between read and write operations are typically not significant. Therefore, the design of the traditional storage system does not take the difference between read and write into consideration. In recent years, as the solid state disk (SSD) develops rapidly, the SSD gradually replaces the traditional mechanical disk. In the SSD, read and write speeds are greatly distinct. As a result, if the difference between read and write is still not considered, severe influences may be caused on the performance of the storage system.